29. Trin. 1.2.4–3.5 (CCL 50:31–32); trans. McKenna, FOTC 45:7–8 (modified).
fesses, believes, and understands that the Father and the Son and the Holy Spirit are of one and the same substance or essence so that [believers] may not feel mocked, as it were, by our excuses, but may learn from actual experience that the supreme Good exists, which can be discerned only by minds that are wholly purified, and that they themselves are unable to see it or to comprehend it for this reason: because the weak eye of the human mind cannot be fixed on a light so dazzling unless it has been nourished and become stronger by the justice of faith. But we must first find out by an appeal to the authority of the sacred Scriptures whether faith is in a position to do so. Next, if God is willing and grants us His help, we shall perhaps render such a service to these garrulous disputants—more arrogant than capable, and therefore suffering from a dangerous disease—that they may discover something which they cannot doubt. But if no such certainty can be found, then they will grumble at their own minds rather than at the truth itself or at our explanations. And thus if there is any spark of the love or fear of God in them, they may return to the principles of faith and to the right course, and may at last realize how wholesome is the medicine that has been entrusted to the holy Church for the faithful. By it a carefully-regulated piety heals the weakness of the mind in order that it may perceive the unchangeable truth, lest some disorderly rashness plunge it into beliefs that are harmful and false. As for myself, I shall not be reluctant to make inquiries if I am anywhere in doubt, nor shall I be ashamed to learn if I am anywhere in error.
(3.5) Therefore, let everyone who reads these pages proceed further with me, where he is as equally certain as I am; let him make inquiries with me where he is as equally hesitant as I am; wherever he recognizes the error as his, let him return to me; wherever it is mine, let him call me back. Thus let us enter together on the path of charity in search of Him of whom it is said: "Seek His face always" (Ps 104:4). This is the sacred and the secure agreement into which I, in the presence of the Lord our God, shall enter with those who read what I am writing, in all my writings, and especially the present one, where we are investigating the unity of the Trinity, of the Father and the Son and the Holy Spirit. For nowhere else is the error more dangerous nor the search more laborious nor the discovery more fruitful.29
On the Trinity: Augustine's Overview
In his letter to Aurelius, Augustine stressed that individual "books are linked to the preceding ones by a continuous development of the argument." Despite Augustine's stress,
29. Trin. 1.2.4–3.5 (CCL 50:31–32); trans. McKenna, FOTC 45:7–8 (modified).